E.M. Castillo1, K.E. Bradbury1-3, A. W. Betts1, N. Charkoudian2, A.T. Lovering1

1University of Oregon, Eugene, OR; 2United States Army Research Institute of Environmental Medicine, Natick, MA; 3ARCS program, Oregon Chapter

During exercise in the heat, approximately 14 - 20% of heat loss occurs via respiration, termed respiratory heat loss (RHL). RHL involves components of both evaporative (Eres) and convective (Cres) heat loss from the upper respiratory tract. Previous research has shown that men with a patent foramen ovale (PFO) have higher core temperatures (Tc) at rest and during exercise. It is unknown whether differences in RHL contribute to the differences seen in Tc between PFO+ and PFO- men. PURPOSE: The purpose of the study was to test whether there are differences in RHL (Eres, Cres, and total RHL (Tres)) between PFO+ and PFO- men at rest and during 60 min of exercise at a workload eliciting a heat production (Hprod) of 7 W/kg. METHODS: Twenty one healthy males (11 PFO+, 10 PFO-, 18-36 y/o). Visit 1 included an ultrasound screening to test for the presence or absence of a PFO. Visit 2 involved a graded exercise protocol to determine the workload that would elicit a heat production of 7 W/kg, followed by a VO2peak test. During the graded exercise protocol, subjects cycled at 4 different workloads for 5 min each. For visit 3, subjects completed 60 min of cycling exercise at a previously determined workload eliciting a Hprod of 7 W/kg in a thermoneutral laboratory environment (22°C, 39% rh). Minute ventilation (VE) and inspired and expired temperature and humidity were measured. RHL was calculated at baseline (BL) and during min 0-10, 25-30, and 55-60 of exercise. Tc was measured using a telemetric pill ingested ~10 hr prior to testing. RESULTS: There were no differences in RHL (Cres, Eres, or total RHL) between PFO+ and PFO- men at rest or during exercise (p>0.05). Using a two-way ANOVA (Tres RHL X Exercise), there was a main effect of exercise on RHL (p<0.01), with RHL being greater at all 3 time points compared to rest and at min 55-60 vs min 0-10 (p < 0.01). Tc was significantly higher in PFO- vs PFO+ men at rest and during exercise (PFO- 37.13 ± 0.18 °C, PFO+ 36.89 ± 0.19 °C at rest vs. PFO- 37.62 +/- 0.16°C, PFO+ 37.44 ± 0.16°C during exercise; p < 0.05). CONCLUSION: While RHL increased from rest to exercise due to increases in VE and metabolic heat production, RHL is not likely the mechanism to explain the differences in Tc seen between PFO+ and PFO- men. Why the PFO- subjects had a higher core temperature in this study remains unknown.

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